This is not the first time there is a phone that has two front cameras. However, if it is only used for a selfie, it seems that the use of two cameras this is just a waste, according to a report from Ubergizmo.
But based on the results of dismantling the APK has recently seen a feature named Face Match which will probably be available in Pixel 3. Face Match facial recognition feature is to unlock the phone, and it looks like will make use of two cameras that front.
The presence of this facial recognition feature starting in the market after Apple introduced the Face ID. If Face Match is a feature of challengers Face ID, this may also be the reason Google to use two front cameras in Pixel 3.